Centaurus A is one of the busiest galaxies yet discovered, as depicted in this combined optical, X-ray, and radio image. The optical image shows dark lanes of dust across the center of the galaxy, which are probably the result of a collision with a smaller galaxy. These regions are giving birth to thousands of new stars. The X-ray image shows great plumes of gas shooting out from the galaxy's center (blue), propelled by magnetic fields around a supermassive black hole. The jets of gas span tens of thousands of light-years. As the gas rams into the thinner gas between stars, it cools and emits copious amounts of radio waves (orange). [NASA/CXC/CfA/R.Kraft et al.; MPIfR/ESO/APEX/A.Weiss et al.; ESO/WFI]

Some galaxies are calm and serene. But others emit copious amounts of radio waves -- a sure sign that they’re anything but calm. The nearest such galaxy is in the southern constellation Centaurus, about 12 million light-years from Earth.

Now known as Centaurus A, the galaxy was discovered by a man born in a country located so far north that its inhabitants never see it. But in the early nineteenth century, Scottish astronomer James Dunlop traveled to Australia, where he scanned the southern skies with a small telescope. On the night of April 29th, 1826, in the 10 o’clock hour, Dunlop spotted a nebula that seemed to have two parts. He had no idea that it was a galaxy beyond our own.

More than a century later, other astronomers in Australia detected radio waves from the constellation Centaurus. The following year, these astronomers moved to a better site in New Zealand and found that the radio waves came from the galaxy Dunlop had seen in 1826.

Today, we know quite a bit about Centaurus A. It’s an elliptical galaxy -- a type that normally lacks gas and dust and the young stars they produce. But Centaurus A has probably swallowed a smaller galaxy, which splashed dust across it. This extra dust creates a dark lane that seems to split the galaxy in two, explaining why its discoverer thought it was really two objects sitting side by side. The collision also caused the radio waves -- making Centaurus A the nearest radio galaxy to Earth.